Technical Field
This invention relates to lattice tower for actuate under high load conditions, more particularly to lattice towers utilized for wind turbines and other applications.
Background Art
Vertical structures for supporting high loads such as towers or the like utilized for supporting wind turbines, power transmission lines and other applications are well known in the prior art. The structural designs, components and materials of such vertical structures vary depending upon the application.
One type of vertical structure that has been receiving special attention in the last decades are the vertical structures for wind turbines or other high loads.
Wind energy has become a very attractive source of energy, both due to an increase in efficiency of the generators and an increase in market demand for clean and renewable sources of energy. The increase of the efficiency of the wind energy generators is related to a great effort in enhancing several aspects of the technology, including many issues related to the design and manufacturing of the wind energy generator components including, among others, the rotor blades, the electrical generator, the tower and the control systems.
Most wind turbines used in megawatt applications, nowadays varying in the range of about 1 MW to 5 MW, have a horizontal-axis wind turbine (HAWT) configuration with a main rotor shaft and an electrical generator at the top of a tower, and the rotor axis directioned to the inflow of the wind with three-blades positioned upwind.
The main advantage of the upwind design is the avoidance of the wind shade and resulting turbulence behind the tower. Currently, most of large scale wind turbines adopt the upwind design; however, this design has various drawbacks such as the need of some distance between the tower and the blades due to the bending of the blades and the need of a yaw mechanism to keep the rotor facing the wind. The yaw mechanism usually has a wind sensor associated by an electronic controller to a yaw drive, which includes one or more hydraulic or electric motors and a large gearbox for increasing the torque, as well as a yaw bearing. The yaw bearing provides a rotatable connection between the tower and the nacelle of the wind turbines. The yaw mechanism usually includes additional components, such as brakes that work in cooperation with the hydraulic or electric motors in order to avoid wear and high fatigue loads on the wind turbine components due to backlash during orientation of the rotor according to the wind direction. As the wind turbine will usually have cables that carry the electric current from the electric generator down through the tower, the cable may become twisted due to the rotation of the yaw mechanism. Therefore, the wind turbine may be equipped with a cable twist counter that is associated with the yaw mechanism electronic controller in order to determine the need of untwisting the cables by the yaw mechanism.
The downwind design, by which the rotor is placed on the lee side from which the wind blows in tower, would in principle avoid the need of a yaw mechanism if the rotor and nacelle have a suitable design that makes the nacelle follow the wind passively, utilizing the wind force in order to naturally adjust the orientation of the wind turbine in relation to the wind. This theoretical advantage is doubtful in large megawatt wind turbines because there usually remains a need to untwist the cables if the rotor continuously turns in the same direction. In addition, there are mechanical problems such as fatigue of the components due to strong loads resultant from the sudden changes of the wind direction. Nevertheless, the downwind design still presents an important advantage in regard to the structural dynamics of the machine, allowing a better balancing of the rotor and tower. In the case of larger wind turbine rotors, which nowadays have a diameter reaching about 120 meters (about 393.6 ft) or more, obtaining more flexibility in the design of the rotor blades is essential.
However, the increase of diameter of the rotor usually involves heavier rotors and the increase of the height of the tower, consequently, may involve the use of additional material, for instance, steel, for manufacturing the tower.
Hence, as a tower usually represents about fifteen to thirty percent of the cost of the wind energy generator, there is a great need to obtain higher and more robust towers at lower costs.
Most large wind turbines manufactured in the last two decades with a power output higher than one megawatt adopt tubular steel towers, commonly referred to as “monopoles”, as the preferred choice. The monopoles usually taper from the base to the top or close to the top, having modules connected together with bolted flanges. A constraint related with monopoles is the road transportation limitations that restrict the diameter of the segments. For instance, tubular segments with diameters higher than about 4 meters (about 13 feet) may not be transported on roads in many countries.
Lattice towers usually need less material (e.g. less steel) than monopoles, but require a higher number of components and bolted connections. These bolted connections are subject to the varying fatigue loads, hence, they have the disadvantage of higher maintenance needs.